Field of the Invention
The present invention relates to a stepping motor.
Description of the Related Art
A stepping motor is used as a drive source of a movable member of various kinds of apparatuses, and it is especially used as a drive source of an optical adjusting member such as a lens that requires position control accuracy. Recently, according to a design which reduces a size of a camera or a lens, a small-size stepping motor that further increases space efficiency and that has high-drive torque characteristics is required. In the stepping motor, silent characteristics that are appropriate for taking a moving picture of the camera, as well as the improvements of drive torque characteristics, are also required. Therefore, various kinds of proposals to reduce a noise of the stepping motor itself are made.
Japanese Patent Laid-Open No. H8-149780 discloses a stepping motor in which one of both ends of bearings is extended to abut against a rotor magnet so as to displace an abutment surface of coil bobbins from a center surface of the rotor magnet in an axial direction by a required amount. In the stepping motor of Japanese Patent Laid-Open No. H8-149780, magnetic centers of the rotor magnet and the coil bobbin are previously displaced from each other by the required amount. Therefore, even when a magnetic balance between the rotor magnet and the coil bobbin is lost in accordance with an individual difference of parts, the displacement of the rotor in the axial direction is prevented. Therefore, a hitting sound that is generated by the rotor magnet and the bearing can be decreased.
However, in the stepping motor of Japanese Patent Laid-Open No. H8-149780, in order to prevent the displacement of the rotor in the axial direction due to losing the magnetic balance between the rotor magnet and the coil bobbin, the magnetic centers of the rotor magnet and the coil bobbin are displaced. Therefore, compared to a case where magnetic centers of the rotor magnet and the coil bobbin are coincident with each other, the drive torque of the motor is reduced. On the other hand, in order to ensure a desired drive torque in a configuration of Japanese Patent Laid-Open No. H8-149780, a total length of the motor needs to be extended by an amount of compensation for the reduction of an opposed area of the rotor magnet and the coil bobbin caused by extending the bearing, and thus the size of the stepping motor is increased.